1. Field of the Invention
The present invention relates to a thin film magnetic memory device, and more particularly, to a thin film magnetic memory device suitable for drive by a battery.
2. Description of the Background Art
Recently, a magnetic random access memory device (MRAM device) has attracted attention as a high-speed and highly integrated non-volatile memory. The MRAM device represents a memory device using a plurality of thin film magnetic elements formed on a semiconductor integrated circuit as a memory cell permitting random access thereto.
Particularly, it has been disclosed that a performance of the MRAM device is enhanced by employing a memory cell constituted of thin film magnetic elements utilizing magnetic tunnel junction (MTJ) (hereinafter, also referred to as an “MTJ memory cell”). The MTJ memory cell stores data, by being magnetized in a direction in accordance with write data by a magnetic field produced by a data write current. Data reading from the MTJ memory cell is carried out by utilizing such a characteristic of the MTJ memory cell that an electric resistance of the MTJ memory cell is varied in accordance with the magnetic direction. Since the magnetic direction of the MTJ memory cell that has once stored data is held until another magnetic field is applied and the data is rewritten, the MRAM device is capable of non-volatile data storage (for example, see U.S. Pat. No. 6,034,887; “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell,” by Roy Scheuerlein et al., 2000 IEEE ISSCC Digest of Technical Papers, TA7.2, “Nonvolatile RAM based on Magnetic Tunnel Junction Elements,” by M. Durlam et al., 2000 IEEE ISSCC Digest of Technical Papers, TA7.3; and “A 256 kb 3.0V 1T1MTJ Nonvolatile Magnetoresistive RAM,” by Peter K. Naji et al., 2001 IEEE ISSCC Digest of Technical Papers, TA7.6).
In addition, a technology for achieving higher integration by covering a metal wire through which a data write current flows with a thin ferromagnetic material in an MRAM device has also been reported (for example, see “A 1-Mbit MRAM Based on 1T1MTJ Bit Cell Integrated With Copper Interconnects,” by Mark Durlam et al., IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 38, NO. 5, MAY 2003, pp. 769-773).
As the MRAM device is excellent in terms of a degree of integration and the number of permitted times of rewriting, portable application of the same such as portable electronics including a portable phone and an IC card has been expected. With regard to portable use, as it is difficult to constantly supply electric power from an outside source, limited electric power supply from a battery or the like is performed.
As disclosed in the documents listed above, however, in the MRAM device, a pulse-like current (data write current) is consumed in order to produce a magnetic field for data write. Therefore, a load current to the MRAM device supplied from a power supply represents a high-frequency, pulse-like current. In general, such a load current pattern accelerates battery exhaustion. Therefore, if the MRAM device is applied to portable use, a period for electric power supply from the battery may be shortened.
In addition, it is difficult to ensure electric power capacity of the power supply in an application such as an IC card or the like. Therefore, it is necessary to ensure a sufficient data write current level within the MRAM device serving as a load under such a condition that a peak value of the load current supplied by the power supply is restricted. Moreover, in a portable application, a smaller circuit area of the MRAM device is desirable.